The liveCD is built with the current stable portage tree at the time of the build, so its gcc build version varies with time.
mdadm only take the static USE flag and that must be on for use in an initrd as no libraries or dynamic linker will be available.

If you mean what gcc options were used to build mdadm on the liveCD, you will need to ask the release engineering team.
The best way to reach them is an email to their alias, or ask in #gentoo-releng on irc.freenode.net_________________Regards,

NeddySeagoon

Computer users fall into two groups:-
those that do backups
those that have never had a hard drive fail.